Exhaust gases emitted from internal combustion engines such as automobiles include harmful components, such as hydrogen carbide (BC), carbon monoxide (CO), and nitrogen oxides (NOx). Three-way catalysts have conventionally been used to detoxify the exhaust emissions.
Conventional three-way catalysts have a noble metal such as, Pt, Pd, and Rh as a catalytically active component, and a carrier such as, alumina, ceria, zirconia, and ceria-zirconia composite oxide having oxygen storage ability. As a catalyst substrate, a ceramic or metallic material having the form of honeycomb, plate, pellets, and the like is used.
With the ever tightening of car emission control, the demands for Pt and Rh, which are the main catalytically active components of catalysts for purifying emissions from inert combustion engines, have been increasing, and their prices have been escalating. In particular, the price of Rh is largely varying. Therefore, using Pd, which is less expensive than other noble metals, in substitution for Rh has been desired. It has been studied to use relatively inexpensive Pd as a catalytically active component thereby to reduce the cost of catalysts for exhaust gas purification. Various means have been proposed in this regard. Among them is the proposal to use aluminum borate as a carrier (see Patent Literature 1 below). According to Patent Literature 1, a pellet of hollow aluminum borate powder having a shell of aluminum borate whisker is used as a carrier, on which a catalyst component is supported. Because the catalyst proposed has the improved gas diffusibility, it exhibits more excellent catalytic performance. However, aluminum borate whiskers described above have a needle shape and thus the whiskers have a small specific surface area, and cannot avoid a problem that the long time use causes the aggregate of the precious metal, as a catalyst for exhaust gas purification. As a solution of this problem, the inventors of the present invention previously developed a catalyst using specific aluminum borate as a carrier (see Patent Literatures 2 and 3 below).